The number of patients suffering from circulatory diseases, such as cardiac infarction or brain infarction, is increasing recently, and prevention and treatment of these diseases has posed a big problem.
Arterial sclerosis has a close relationship with the onset of cardiac infarction and brain infarction. Specifically, when an atheroma is formed on an arterial wall or when new cells of an arterial wall are created for various reasons, such as high blood pressure, the artery loses elasticity, to thus become hard and fragile. As a result of a blood vessel being closed at a position where the atheroma is formed or a vascular tissue covering the atheroma becoming ruptured, the atheroma flows into the blood vessel, to thus close another location of the artery or rupture a hardened region of the artery. In this way, the diseases are induced. Therefore, early diagnosis of arterial sclerosis becomes important for prevention or treatment of these diseases.
Manifestation of arterial sclerosis has recently been diagnosed by means of direct observation of an internal state of the blood vessel through use of a vessel catheter. However, this diagnosis involves a problem of a necessity for insertion of a vessel catheter into a blood vessel, thereby imposing heavy load on a subject. Therefore, observation involving the use of the vessel catheter is employed for a subject who is sure to have a lesion into which arterial sclerosis has developed, to thus locate the lesion. This method has never been used as; for example, a test for health care.
Measurement of a cholesterol level, which is one cause of arterial sclerosis, or a blood pressure level is a test which imposes a small burden on the subject and which can be practiced readily. However, these values do not directly indicate the degree of arterial sclerosis.
So long as arterial sclerosis can be early diagnosed and a therapeutic medicine for arterial sclerosis can be administered to the subject, the medicine becomes effective for treatment of arterial sclerosis. Once arterial sclerosis has progressed, further progress of arterial sclerosis can be inhibited by the therapeutic medicine. However, complete recovery of a hardened artery is said to be difficult to be made.
For these reasons, there has been sought a diagnostic method or a diagnostic apparatus which diagnoses arterial sclerosis in its early stage with involvement of a small burden on the subject before it progresses.
In the meantime, an ultrasonic diagnostic apparatus and an X-ray diagnostic apparatus have heretofore been used as a noninvasive medical diagnostic apparatus which imposes a small burden on the subject. Geometrical information about the inside of a body or information about a chronological change in the internal shape of the body can be acquired without causing pain in the subject by radiation of an ultrasonic wave or an X ray onto the body from the outside. As a result of acquisition of the information about a chronological change in the shape of an object of measurement in the body (i.e., motion information), information about the character of the object of measurement can be obtained. Specifically, an elastic characteristic of the blood vessel in the body can be determined, and the degree of arterial sclerosis can be ascertained directly. In particular, when compared with the X-ray diagnosis, the ultrasonic diagnosis enables performance of measurement by means of bringing an ultrasonic probe into contact with the subject. Accordingly, the ultrasonic diagnosis is superior because it obviates a necessity for administration of a contrast medium or a risk of X-ray exposure.
Recent advancement of electronics technology enables remarkable enhancement of the accuracy of measurement of the ultrasonic diagnostic apparatus. In association with this, development of an ultrasonic diagnostic apparatus which measures micromotion of biomedical tissue is in progress. For instance, use of a technique described in Patent Document 1 enables high-precision measurement of a vibrational component of vasomotion whose amplitude is of several microns and which is as fast as several hundreds of hertz, and hence high-precision measurement of a change in the thickness of the vascular wall or the distortion of the wall to an order of microns is reported to become practicable.
Patent Document 1 discloses a technique for noninvasively measuring a waveform of motion speed of each location in biomedical tissue through use of an ultrasonic wave, thereby determining an elastic modulus of a microarea. Since use of a tissue tracking technique described in Patent Document 1 enables high-precision measurement of vasomotion, a chronological change h(t) in the thickness of an arterial wall can be measured high accuracy. Provided that the thickness of the arterial wall acquired at the time of measurement of low blood pressure is “h”; that the maximum amount of change in the thickness of the arterial wall acquired in one cardiac cycle is Δh; and that pulse pressure is Δp, a radial elastic modulus E of an arterial wall is determined as follows:E=Δp·h/Δh  (Eq. 1)
Use of such a high-precision measurement technique enables detailed measurement of a two-dimensional distribution of elastic characteristics of the arterial wall. For instance, Non-Patent Document 1 describes an example illustration showing a two-dimensional distribution of elastic modulus of a carotid arterial wall superimposed on a B-mode tomogram. The degree of hardness of the arterial wall is not uniform, and the hardness of the arterial wall is present in the form of a certain distribution. Accurately grasping a localized distribution of elastic modulus, which is a feature quantity representing the degree of hardness of an artery, is important for diagnosing arterial sclerosis.
Patent Document 1: JP-A-10-5226
Non-Patent Document 1: Hiroshi Kanai et al., “Elasticity Imaging of Atheroma With Transcutaneous Ultrasonic Preliminary Study,” Circulation, Vol. 107, pp. 3018 to 3021, 2003.